Process of chromizing air hardening tool steel



Jan. 19, 1960 G. A. SAMUEL ETAL 2,921,877

PRocEss oF CHROMIZING AIR HARDENING Toor. STEEL Filed March l5, 1958 United States Patent @eine 2,921,877 Patented Jan. 19, 1960 PROCESS F CHRGM'IZEJG AIR HARDENING TOOL STEEL George A. Samuel, White Plains, N.Y., and Jerome Victor Bell, Newark, Del., assignors, by mestre assignments, to David Craven, Wilmington, Del.

Application March 13, 1958, Serial No. 721,296

3 Claims. (Cl. 148-14) The present invention relates to the hardening and chromizing of air hardening tool steels.

A purpose of the invention is to obtain a hard case on air hardening tool steels.

A further purpose is to give adequate support immediately beneath the case in chromizing on air hardening tool steel, so that the case will not only be very hard, but resistant to abrasion.

A further purpose is to reduce or eliminate decar.

burization of the surface in chromizing air hardening tool steels.

A further purpose is to obtain very effective heat transfer through the chromizing composition and the walls of the retort for the purpose of quenching the air hardening tool steels in the retort.

The drawing illustrates apparatus which may conveniently be applied in performing the process of the invention.

In the prior art, it has been commercially impractical to chromize and then harden air hardening tool steels without largely eliminating the benefit of the chrornizing due to oxidation of the chromium rich surface. If, on the other hand, you chromize after hardening, the reaustenitizing of the steel in the chromizing process destroys the effect of the hardening, or the chromizing temperature draws the steel to such an extent that it is too soft.

We have discovered that austenitizing and chromizing can be carried out together in the chromizing retort, and by using a granular source of chromium surrounding the work and extending to the wall of the steel or heat resisting metal retort, the heat conductivity is adequate so that quenching of the entire retort, while still sealed from the atmosphere, in a liquid medium such as water or oil will properly harden the air hardening tool steel. The parts come out bright when the retort is opened up, and the chromizing is of a very superior quality; being very hard and with the case excellently supported by the metal of the core immediately beneath the case.

When reference is made herein to air hardening tool steels, it is intended particularly to refer to the high carbon, high chromium steels which bear AISI designations of Dl, D2, D3, D4, D5 and D6, and which nominally contain 12 percent chromium and carbon contents between 1 percent and 2.25 percent, with or without 1 percent of molybdenum, or 1 percent of tungsten, and with or without 3 percent of cobalt, the balance being iron, as set forth for example in Metal Progress, Report of 22 Technical Committees of the American Society for Metals, July 15, 1954, at page 22. It is also intended to include the type known as 440C, which contain 16 to 18 percent chromium and l percent carbon, balance iron. It is also intended to include the air-hardening medium alloy tool steels which are designated AISI A2, A4, A5 and A6, and which have chromium contents of 5 percent or l percent, a molybdenum content of 1 percent, carbon content of 0.70 or 1 percent, and in some cases a manganese content of 2 percent or 3 percent, the balance being iron.

We also include under the designation of air-hardening tool steels the molybdenum grades of high speed tool steels which bear AISI designations of M1, M2, M3, M4, M6, M8, M10, M30, M34, V and M36 and which include 4 percent of chromium, with a carbon content varying between 0.80 and l percent, with a vanadium content varying between 1 and 4 percent, a molybdenum content varying between 5 and 8 percent, and in some cases containing between 1.50 and 6 percent of tungsten, in some cases containing between 5 and 12 percent of cobalt, and in one case containing 1,25 percent of columbium, the balance being iron'. Each of these air-hardening tool steels has a recognized austenitizing temperature which is in the range between 1600 and 2400 degrees F. and we conduct the chromizing' at this austenitizing temperature.

The austenitizing temperature is desirably low within the suitable range, as otherwise the steel might come out relatively soft, because of retained austenite. We also prefer to use the shortest possible time for chromizing in order to avoid excessive grain growth. We prefer not to hold the work at the austenitizing and chromizing temperature much longer than two hours, and good results can be obtained in as little as thirty minutes at the austenitizing and chromizing temperature.

In accordance with the invention, the air hardening tool steel parts to be chromized are placed in a retort surrounded by a source of chromium, which is preferably in granular form. By this we mean that it is larger than 40 mesh per linear inch and preferably not exceeding 5% inch diameter, the preferred granular size being in the range between l; inch and 1A inch.

The source of chromium may be ferrochrome, which is preferably the low carbon grade having a carbon content of less than 0.04 percent by Weight, and having a chromium content higher than 65 percent chromium by weight.

In some cases however, granular electrolytic chromium metal will be used instead of ferrochrome.

It is important that the source of chromium be granular so as to obtain most effective heat transfer from the work through the Wall of the retort in quenching.

It is decidedly preferable in the present invention to use a bromide chromizing catalyst, since the bromide ehrornizing catalysts have the property of producing an extremely hard case which is supported by themetal of the core immediately beneath the case. It is much preferable to use ammonium bromide rather than some other bromide like iron bromide or chromous bromide, because ammonium bromide not only acts very effectively as a catalyst, but it does not etch, and it dissociates to expel air from the retort.

Less desirably, we can also use a chloride chromizing catalyst, preferably ammonium chloride, although permissibly another chloride such as iron chloride or chromous chloride, with another material to expel the air. The chlorides are less desirable because of the tendency to etch.

We can also less desirably use an iodide chromizing catalyst, preferably ammonium iodide, although less desirably ferrous iodide or chromous iodide with another compound to expel the air. Here another difficulty is caused by the tendency to etch.

We can also employ a fluoride chromizing catalyst, such as ammonium chromium uoride, ammonium biuoride or ammonium iluoride, but difficulty is encountered through decarburization especially at sharp edges and thin sections, and through the tendency of ammonium bifluoride to etch. Ferrous luoride and chromous iluoride may also be used with another compound to exclude air,

The complex ammonium chromium uoride can conveniently be produced as follows:

Into a solution containing 200 grams of ammonium biuoridedissolved in 200 cc. of-water, metallic chromium 'powder through` 200 mesh per "linear inch is introduced Y until 200grams haverbeen added. Precautions against a violent reaction or explosion are taken by using remote control techniques, since the reaction generates a tremendous amount of heat and if. conducted too rapidly may cause `au explosion.

Thev heat results in drying the product, producing the double uoride of ammonium and chromium Yin the formV Yof'a dry powder. f

' i Example I Into a steel retort havinganinternalvolume of about ,Va of a cubic foot, we placedacharge of 30 pounds of air hardening'tool steelparts of composition 440C mium, in this case, granular ferrochrome, as .above described, of a particle size between 1A; and 1A: inch. The Vcharge in the retort also included 60 grams of ammonium bromide as the chromizing catalyst.V

iThe preferred apparatus of the invention is a steel or heat resisting alloy cylindrical retort20 containing a charge of air hardening tool steel parts 21 surrounded by a screen wire container 22 which is in turn surrounded by a sourceof chromium, `in this case V30 pounds of granular ferrochrome 23 engaging the bottom retort wall 24 and the side retort wall 25, the retort also containing 60 grams of ammonium bromide.

' The retortis surrounded by a muile furnace' 26, suitably electric ,or gas heated, and extends out through the top where it is flanged at 27, the ange being cooled by a water jacket 28, which has water inlet at 30 and water outlet at 31. The top is closed,by a steelfcover 32,- carrying a refractory plug. 33, gasketed at 34 (suitably by silicone rubber or polytetrauorethylene) and "with the gasket clamped by bolts 35 having their heads in the watfer jacket.

'i 2,921,877 f t C. The core very adequately supported the case and the case was resistant against breaking through. Thus, in testing, it was found that you could not break through the case, even at a corner using a iile. You simply dulled the file. The metallographic specimens could not be ground using ordinary techniques,and it was necessary to grind them using diamond dust.`

The thickness of the case wasY Ybetween Y0.0005 and 0.0007 inch. l

' VExample II The procedure of Example IV wascarried on using AISI high carbon, high chromium tool steel B2 having a nominal composition of l2 percent chromium, 1.50 percent carbon andl .percentmolybdenum, balance iron.

The results were similar to those lalready described, the case hardness being Rockwell C equivalent 75.

Example Ill. n Substituting metallic chromium for ferrochrome, it was found that similar results could be obtained using metallicchromium. Y f Y l v VExr'mpla IV The procedure of VExample I is carried out using 60 grams of ammonium chromium-`uoride as Aabove de- A pipe 36 connects with the interior ofthe retort through the cover,rand has a valve 37 and extendsat its outer end beneath the level of water 38 in container 40. A The lower end o f the pipe is preferably connected by rubber hose 41 to glass tube 42 so that suck-back of water can be noticed. l

The workis heated up to temperature while the chrocatalyst dissociates and expels the air, the chromizing, in thiscase, being conducted at 1350u F. At about the timeV the work comes to chromizing temperaturey a negative pressure in the retort develops and the Yvalve 3'7is closed.V AT he work'is held at chromizing'temperature, insthis caseY for two hours after the interior reaches 18501-7. The retort and its contents, stil1 Vsealed from the atmosphere, is then taken'out of the furnace and quenched in a tub of water within one minute of thertime thatr therretort leaves thefurnace.

Y After the retort and itscontents are cooled down, approximately to' 300 F., the.' retort *isopened hand" the Workremcived;v i

scribed instead of ammonium biuoride. The work is air hardening tool steel parts of composition 440C. Where the parts are massive, the case .is hard,.of the order of 75 Rockwell C equivalent, butdifficulties are encountered in obtaining hardness in thin sections and sharp edges. t 1 f Exambl V The procedure of xample carried out using ammonium chromium instead .of ammoniumbifiuoride. A

hard case is obtained as inv Example I, but some diiculty is encountered through etching'of machine surfaces.

Example` The procedure of ExampleI is`carried out using ammonium iodide instead of `ammonium biucride., A hard case Vis obtained as in ExamplefI; but some difficulty is encountered through etching of machine surfaces.

It is desirable, according to the invention, rto use bromide activator, particularly rathergthandluoride, as the fluoride gives a softer'caseand acase that is not as well supported in thin sections. VIVBromideY is much preferred also over chloride and iodide becauseit does not etch. "T

The quantity ofV activator will vary with the volume of the retort, and usually vis Vin the range between 0.06 and '1 percent of the Vweightfof chromium in the source of chromium, preferably. about 0.1`percent. In the above examples, a considerable excess of bromide activator was used. v Y i All percentages. givenherin'are by Weight.

In view of ourinvention anddi'sclsre variations and modifications to meet individual vwhimorV particular need will doubtless becomeevident to others'"'skilled in the art, to obtain all or part of the benefits of our invention without copying the process shown, and we, therefore,

The work may be subsequently furtherk heat treated, Y

as forexample by tempering or deep freezing to remove retained austenite. Y -f A `remarkable effect developed. The chromizing case wasjuniform and free from etching, and extremely hard. V

The hardness in the case was equivalent to 75 Rockwell d theccorehardnes/s was eguivalent to 63 Rockwell claim' all such insofaras they fall within the'reasonable spirit and scope of our claims.

Having thus vdescribed our invention,A what we claim as new and desiretosecure by Letters Patent is':

l. The process Vof chromizing air'hardeningtool steels, which comprises placingrin a metallic retortfair hardening tool steel parts o f the class consisting of AISI D1, D2, D3, D4, D5, D6, 440C, A2, A4, A5, A6, M1, M2, M3, M4, M6, M8, M10, M30, M34, M35 and M36,

. along with a source of granular ymetallicl chromium and a halogen chromizing catalyst, closing theretort to the .atFnQSEhefe heating uatha retort ,211151 ,its [feauteau to a hardening temperature for said steel in the range between 1600 and 2400 degrees F. and holding saidrparts at such temperature for at least thirty minutes, and then quenching the retort and its contents in a liquid cooling medium.

2. The process of claim 1, in which the chromizing catalyst is a bromide. Y

3. The process of claim 2, in which the catalyst is ammonium bromide.

References Cited in the le of this patent UNITED STATES PATENTS Samuel Ian. 2, 1951 Samuel Mar. 4, 1958 Becker et al Feb. 24, 1959 FOREIGN PATENTS Great Britain Feb. 11, 1949 Great Britain Dec. 12, 1956 

1. THE PROCESS OF CHROMIZING AIR HARDENING TOOL STEELS, WHICH COMPRISES PLACING IN A METALLIC RETORT, AIR HARDENING TOOL STEEL PARTS OF THE CLASS CONSISTING OF AISI D1, D2, D3, D4, D5, D6, 44OC, A2, A4, A5, A6, M1, M2, M3, M4, M6, M8, M10, M30, M34, M35 AND M36, ALONG WITH A SOURCE OF GRANULAR METALLIC CHROMIUM AND A HALOGEN CHROMIZING CATALYST, CLOSING THE RETORT TO THE ATMOSPHERE, HEATING UP THE RETORT AND ITS CONTENTS TO A HARDENING TEMPERATURE FOR SAID STEEL IN THE RANGE BETWEEN 1600 AND 2400 DEGREES F. AND HOLDING SAID PARTS AT SUCH TEMPERATURE FOR AT LEAST THIRTY MINUTES, AND THEN QUENCHING THE RETORT AND ITS CONTENTS IN A LIQUID COOLING MEDIUM. 